Method of producing a matched parameter acceleration cancelling hydrophone

ABSTRACT

A seismic cable having minimum response to speed changes is provided. The cable includes a plurality of miniature seismic pressure sensitive detectors coupled one to another in such a manner so as to minimize through cancellation undesired electrical disturbance resulting from speed changes. Each detector includes transducers of the ceramic piezoelectric crystal type having matched geometric and physical property parameters. Each ceramic piezoelectric crystal is provided with metal electrodes such as, for example, gold, nickel, platinum, or rhodium which, in addition to being corrosion resistant and inactive as to the constituents in the ceramic material, are insoluble in liquids encountered by the detector in its use environment to avoid dendrite type growth of the metal through minute cracks developing in the ceramic which causes short circuits in the piezoelectric element.

United States Patent 1 Johnston et a1.

[4 1 Sept. 3, 1974 METHOD OF PRODUCING A MATCHED PARAMETER ACCELERATIONCANCELLING HYDROPHONE [75] Inventors: Roy C. Johnston, Richardson;

Lawrence B. Sullivan, Plano, both of Tex.

[73] Assignee: Texas Instruments Incorporated,

Dallas, Tex.

[22] Filed: Dec. 26, 1973 ['21 Appl. No.: 427,881

Related US. Application Data [63] Continuation-in-part of,Ser. No.255,503, May 22,

1972, abandoned.

[52] US. Cl 29/2535, 310/87, 310/91,

340/8 R, 340/10 [51] Int. Cl B0lj 17/00 [58] Field of Search 29/2535;340/10, 8 R,

340/8 PC, 8 FT; 310/83, 8.7, 9.1

3,555,503 1/1971 Morris 340/10 X Primary ExaminerCharles W. LanhamAssistant ExaminerCarl E. Hall Attorney, Agent, or Firm-l-larold Levine;Rene E. Grossman; Alva H. Bandy 5 7] ABSTRACT A seismic cable havingminimum response to speed changes is provided. The cable includes aplurality of miniature seismic pressure sensitive detectors coupled oneto another in such a manner so as to minimize through cancellationundesired electrical disturbance resulting from speed changes. Eachdetector includes transducers of the ceramic piezoelectric crystal typehaving matched geometric and physical property parameters. Each ceramicpiezoelectric crystal is provided with metal electrodes such as, forexample, gold, nickel, platinum, or rhodium which, in addition to beingcorrosion resistant and inactive as to the constituents in the ceramicmaterial, areinsoluble in liquids encountered by the detector in its useenvironment to avoid dendrite type growth of the metal through minutecracks developing in the ceramic which causes short circuits in thepiezoelectric element.

5 Claims, 15 Drawing Figures [56] References Cited UNITED STATES PATENTS2,448,365 8/1948 Gillespie 340/10 3,187,300 6/1965 Brate 340/103,458,857 7/1969 Hancks et a1. 340/10 PATENTED 3EP3 74 SHEET 1 BF 4PATENTED 393 7 snmeord ACCELERATION FORCE PRESSURE FORCE METHOD OFPRODUCING A MATCI-IEI) PARAMETER ACCELERATION CANCELLING HYDROPHONE Thisis a continuation-in-part of application Ser. No. 255,503 filed May 22,1972 now abandoned.

BACKGROUND OF THE INVENTION I. Field of the Invention This inventionrelates to an improved hydrophone and more particularly to an improvedpressure sensitive detector element therefor.

2. Description of the Prior Art Seismic cables utilized in water workare commonly referred to as streamers. In a streamer the cable anddetectors are built into a single piece of equipment, sections of whichare joined before they are placed in the water. The streamer is handledby a large powerful winch mounted on the stern of a survey ship. Eachsection includes a tube and up to 30 detectors or more. The tube, whichis generally constructed of a heavy duty plastic, is made neutrallybuoyant by being filled with a liquid, e.g., kerosene or a plastic foam.Kerosene also provides very effective acoustic coupling from marinepressure changes to the detectors. The tension load is carried by steelwire rope strain members running the length of the streamer. Conductorsare provided for feeding seismic information generated by the detectorsto data recording equipment and depth information from depth transducersto a depth meter console. The signal-to-noise ratio is the ratio of theamplitude of a desired seismic signal at any time on the seismic traceto the amplitude of undesired electric disturbance (noise) signals atthe same time on the trace.

The problem of enhancing the signal-to-noise ratio has long confrontedthe art; efforts to enhance their ratio have been made. These effortsresulted in a method of supporting the detectors within the plastic tubeto make them susceptible to random transverse accelerations anddecelerations induced by towing, and to include, in detectors usingpiezoelectric transducers, an impedance matching step-down transformerfor each detector group. This latter improvement is necessary becausethe output impedance of a group of such detectors is usuallyunacceptably high as an input to ordinary seismic amplifiers and isundesirable because of noise pickup problems.

Pressure-sensitive detectors utilize piezoelectric elements whichgenerate an electromagnetic force (emf) when stressed by an externalforce applied to their surface. In the past, pressure-sensitivedetectors or hydrophones employed as their piezoelectric elementsanistropic crystals such as quartz or rochelle salt. More recently,however, these anisotropic crystals have been replaced bypolycrystalline and isoptropic ceramic materials. An anisotropicsubstance exhibits different properties in different directions; whileisotropic substances exhibit the same properties (e.g. electrical oroptical) in all directions.

The piezoelectric elements or transducers used in hydrophones aredesired to operate over a relatively large frequency range which extendsup to several octaves below the devices resonant frequency. Thus theyare often referred to as nonresonant transducers. The transducerscomprise a ceramic dielectric having metal electrodes formed on eachmajor surface. The electrical equivalent circuit for a nonresonanttransducer can be approximated by an ideal voltage generator in serieswith a capacitor. The capacitor represents the electrical capacitancebetween the electrodes, and its value depends on the physical dimensionsof the piezoelectric element and the dielectric constant of theparticular type of ceramic used. The capacitance of the typicalpiezoelectric element used today ranges from 10 to I00 nanofarads. It issufficient for piezoelectric elements used in hydrophones or seismicdetectors to have a capacitance ranging from 9 to 11 nanofarads (nf)since using 30 such detectors in parallel results in a minus 3 dB pointof approximately 6 Hz for the section. To obtain this value thethickness of the ceramic or dielectric material of the piezoelectricelement must be substantially reduced. Efforts to reduce the thicknessof the dielectric of prior art ceramic type piezoelectric elements oftransducers which utilize as electrodes metals capable of supportingdendrite type growth such as, for ex- I ample, silver failed in usebecause electrical short circuits developed between the electrodeplates. It has been determined that these electrical shorts were theresult of silver of the silver electrodes migrating through cracks inthe ceramic dielectric through what is believed to be a dendrite growthtype action.

SUMMARY OF THE INVENTION It is an object of the invention to provide animproved seismic cable or streamer.

It is another object of the invention to provide an improvedpiezoelectric element for improving the lifetim of a pressure typeseismic detector.

It is a further object of the invention to provide a detector and aseismic cable or streamer having respectively first and second orders ofacceleration and deceleration noise cancellation.

Still another object of the invention is to provide a detector havinghigh sensitivity.

Yet another object of the invention is to provide an improved method ofmanufacture of a detector having a high sensitivity.

The above objects and other objects of the invention are accomplished byfabricating piezoelectric crystals or elements for detector elements,selecting two detector elements having matched geometric andphysicalproperty parameters for each pressure-sensitive detector toreduce the detectors sensitivity to acceleration and deceleration noise(hereinafter referred to as acceleration noise), and by constructing aseismic cable or streamer having its pressure-sensitive detectorsconnected one to the other with the polarity of one reversed as to thepolarity of the other for the same acceleration noise. Thus theacceleration noise of one detector, being out of phase relative to theother, subtracts or cancels the acceleration noise of the other detectorto enhance the signal-to-noise ratio of the seismic signals generated bythe cable.

The seismic cable may be constructed in different ways; however, thepreferred construction will be described hereinafter in conjunction withthe drawings.

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG. 1 is a side view of a seismic streamer being towed through water;

FIG. 2 is a perspective view of a miniature detector constituting anembodiment of this invention;

FIG. 2A is an isometric view of a miniature detector constitutinganother embodiment of this invention;

FIG. 3 is a perspective view of the miniature detector subassembly;

FIG. 4 is a sectional view of the miniature detector taken along lines4-4 of FIG. 2;

FIG. 5 is a schematic view of the miniature detector and showing indotted lines its response (greatly exaggerated) to pressure forces;

FIG. 6 is a view similar to FIG. 5 showing the miniature detectorsresponse (greatly exaggerated) to acceleration forces;

FIG. 7 is a perspective view of the piezoelectric element;

FIG. 8 is a diagram of the equivalent electromechanical circuit of thedetector of FIG. 4;

FIG. 9 is a schematic diagram showing a pair of hydrophones of FIG. 4arranged in a seismic streamer with the polarities as shown;

FIGS. 9A and B are waveforms of the voltage outputs of the hydrophonesas arranged in FIG. 9 due to the acceleration force;

FIG. 10 is a schematic diagram showing the same pair of hydrophones ofFIG. 9 with one of the hydrophones physically oriented 180 from itsposition of FIG. 9;

FIGS. 10A and B are waveforms of the voltage outputs of the hydrophonesas arranged in FIG. 10 due to the same acceleration force (FIG. 9).

Referring to the drawings, a seismic streamer 10, having a plurality ofsections 12, is towed through the water at a constant depth by a ship 14(FIG. 1). The ship 14 has a winch 16 for laying and retrieving theseismic streamer 10. The streamer 10 is made up of one or more tubularsections joined together by connectors, not shown, called integratedcouplers. These couplers provide electrical continuity for all theconductor pairs from the detectors, hereinafter described, and the depthtransducers, and water-break transducers (not a part of the presentinvention). In addition the couplers provide mechanical coupling for thestress members running from one end to the other of each section. Eachsection contains the required number of conductor pairs necessary tocarry all the hydrophone-group circuits into the instruments. Normallyup to 70 pairs of conductors or more are provided, 54 of which are forseismic data, 4 are for spares, 6 are for the depth transducers and 6are for the water-break transducers.

Each section 12 contains a plurality of hydrophones arranged in thestreamer 10 in accordance with the second order acceleration noisecancellation embodiment of the invention. Each hydrophone includesdetector elements 58 and 58' (FIGS. 5 and 6) constructed and combined toform a hydrophone subassembly 22 having a first order acceleration noisecancellation mode hereinafter described.

It will be understood that ideally two detector elements 58 and 58'(FIG. 4) are used in each hydrophone and they are to have the samesensitivity to acceleration so that the acceleration noise will becancelled completely and the response of each detector element topressure forces will be substantially equal. This desired result can beclosely approximated by matching geometric and physical propertyparameters and electrical properties of elements of the hydrophone priorto and during the construction. Parameters which can be changed aredetermined by equating electrical units to mechanical units inaccordance with the mathematical relationship derived from an equivalentelectrome- Ill chanical circuit. In deriving the mathematical formulathe electrical units and analogous mechanical units set forth in TABLE Iare utilized.

TABLE I Electrical Unit Analogous Mechanical Unit Voltage Force CurrentVelocity Charge Displacement Capacitan ce Compliance Inductance MassImpedance Mechanical Impedance Each ceramic piezoelectric element issubjected during use to both acoustic wave (pressure) forces andacceleration noise forces; and equivalent electrical circuit for theseconditions is shown in FIG. 8. The pressure forces are presented in themechanical side by F (t), and the acceleration noise forces arerepresented by velocity perturbations V,,(t). The mass of eachpiezoelectric element is M, the compliance of its mounting is C,,,. Theelectromechanical transformer has a turns ratio of N, and thecapacitance C, and output voltage E, are shown on the electrical side.The impedances of the detector elements 58 and 58' are representedrespectively by Z and Z The equivalent circuit is divided into amechanical side and an electrical side by the electromechanicaltransformers 70 and 72 for the first detector element 58 and theelectromechanical transformers 74 and 76 for the second detector element58. The electromechanical transformers represent the electrical responseto the force applied to the piezoelectric elements 44, 46, 48 and 50,(FIG. 4) respectively. The transformers 70, 72, 74 and 76 (FIG. 8) haveturn ratios respectively N 1, N 1, N 1, and N 1. The mechanical side ofthe equivalent circuit for the first detector element 58 includes aforce generator 78 and a velocity generator 80 which are equivalentrespectively to the pressure forces (F )t and the velocity disturbanceperturbations V (t). The positive terminals of these generators 78 and80 are coupled to an inductor 82 which represents the mass (M of thepiezoelectric electric element 44. The inductor 82 is coupled to one endof a parallel circuit comprising a capacitor 84 which represents thecompliance of the piezoelectric element 44, coupled across the primaryof transformer 70. The other end of the parallel circuit is coupledthrough a capacitor 86 to 5 an inductor 88 representing respectively thecompliance (C and mass (M,,) of an electrically conductive diaphragm 40(FIG. 4). Inductor 88 is coupled to an inductor 90 (FIG. 8) whichrepresents the mass (M of piezoelectric element 46. The inductor 90 isin turn coupled to one end of a parallel circuit comprising a capacitor92 which represents the compliance of the piezoelectric element 46coupled across the primary winding of transformer 72. The other end ofthe parallel circuit is attached to the other end of the generators 78and to complete the mechanical side of the first detector element. Theelectrical side of the first detector element includes the followingelements coupled end to end or in series: the positive terminal,capacitor 94 which represents the electrical capacitance (C ofpiezoelectric element 44, the secondary of electromechanical transformer70, capacitor 96 which represents the electrical capacitance (C of thepiezoelectric element 46, the secondary of electromechanical transformer72, and negative terminal 28. The output of the first detector elementis designated E The mechanical and electrical sides of the equivalentcircuit of the second detector element 58 are substantially identical tothose of the first detector element 58. The principal differences arethat piezoelectric elements 48 and 50 and diaphragm 42 replace thepiezoelectric elements 44 and 46 and diaphragm 40. Thus the mechanicalside includes a force generator 98 and velocity perturbation generator100 which represent respectively the pressure forces F(t) andacceleration forces V,,(t) exerted on the second detector element 58.Positive terminals of the generators 98 and 104 are connected through aninductor 102 which represents the mass (M of the piezoelectric element50 to one end of a parallel circuit having a capacitor 164 whichrepresents the compliance of piezoelectric element 50 coupled across theprimary of the electromechanical transformer 74. The other end of theparallel circuit is attached to an inductor 106 which in turn isconnected to a capacitor 108 which represent respectively the mass (Mand compliance (C of the diaphragm 42. The capacitor 108 is coupledthrough an inductor H0 representing the mass (M of piezoelectric element48 to one end of a parallel circuit comprising a capacitor 112 whichrepresents the compliance of piezoelectric element 48 coupled across theprimary of electromechanical transformer 76. The other end of theparallel circuit is connected to the other end of the force and velocitygener- B. The voltage output owing to an acceleration disturbance on thesecond detector comprising piezoelectric elements 48 and 50 is C. Thecombined open-circuit, output of the detector is given by ators 98 and100 to complete the mechanical side of Wherei n and 12 are respectivelythe electromechanthe second detector element. The electrical side of thesecond detector element comprises the following elements connected inseries. From the positive terminal 26 the secondary of electromechanicaltransformer 76, capacitor 114, which represents the electricalcapacitance (C of piezoelectric element 50, secondary ofelectromechanical transformer 74, capacitor 116, which represents theelectrical capacitance (C of piezoelectric element 48, and the negativeterminal 28. The electrical output of the second detector element isdesignated E The following mathematical formulas are derived from theequivalent circuit.

A. The voltage output owing to the same acceleration force on the firstdetector comprising piezoelectric elements 44 and 46 is f w g omar ill Anmaa:

where;

(91112 ml m2)/( l M2 M11): al

angular frequency for detector 58 mounting ical impedance of the firstand second detector elements.

M M M M Referring to FIG. 2 for a description of the first embodiment ofthe invention, the detector or hydrophone construction comprises ahousing 20 for a hydrophone subassembly 22 (FIGS. 2 & 3). The housing 20may be constructed of any suitable shock resistant material such as, forexample, stainless steel and has its major exterior surface coated orotherwise covered with a shock absorbent and electrically resistantsleeve 21 or coating made of rubber or a suitable plastic. One end ofthe housing 20 supports a pair of output terminals 26 and 28 fixedthereto in insulators 29. The inner surface of the housing 241) isprovided with a hydrophone subassembly mounting means 30 (FIG. 4) whichmay be either a groove or a series of spaced holes.

The hydrophone subassembly 22 (FIG. 3) includes a retaining means 32(FIG. 4) adapted to mate when cov- 5 ered by an encapsulating plasticwith the hydrophone subassembly mounting means 30 to position thehydrophone subassembly within the housing 20. The retaining means 32 maybe formed as an integral part of a de tector element frame 34 (FIG. 4).The frame 34 has opposing ends 36 and 38 having recesses which openinwardly to receive respectively electrically conductive diaphragms 40and 42 of detector elements 58 and 58' and to retain them in spacedaxial alignment. The frame 34 may be formed by molding any suitableplastic such as, for example, a polycarbonate plastic which aftersetting has the rigidity to hold the diaphragms with a reproduciblecompliance to pressure forces. The electrically conductive diaphragms 40and 42 may be constructed of any suitable material such as, for example,brass, beryllium, copper, phosphor bronze or other copper alloy metal.The diaphragms are of a thickness of about 0.016 inches, the thicknesscriterion being that the diaphragms have sufficient strength towithstand expected hydrostatic pressures yet deform sufficiently in anacoustic pressure field to generate adequate electrical signals. Thediaphragms 40 and 42 have major opposing surfaces adapted to receiverespectively piezoelectric elements 44 and 46, and 48 and 50 to form thedetector elements 58 and 58.

The piezoelectric elements 44-50 are identical in construction andtherefore only one need be described in detail. Each piezoelectricelement (FIG. 7) comprises a dielectric member 52 constructed ofsuitable ceramic materials such as, for example, barium titanate, leadzirconate, and lead titanate or a mixture of lead zirconate and leadtitanate. Although the dielectric member 52 may be formed in any desiredshape, as shown in FIG. 7, it is extruded in the form of a cylinder andthen sliced into flat circular disks having a substantially uniformthickness of less than about 0.015 inches. Next, dendrite growthinhibitor metal electrodes 54 are formed on opposing sides of theceramic disk 52. The metal must be corrosion resistant, inactive as tothe constituents of the ceramic material, and insoluble or very stableas to liquids encountered by the hydrophone in its use environment toinhibit dendrite type growth of the metal through minute cracksdeveloping in the ceramic and subsequent short circuiting of thepiezoelectric element. A suitable metal is, for example, gold, nickel,platinum, or rhodium with gold or nickel being preferred. Goldelectrodes are formed by depositing a thin layer of gold or gold fritonto the major surfaces of the ceramic disk by a thick film process tobe described. If the piezoelectric elements are to be soldered to asupporting diaphragm 58, a border 56 (FIG. 7) is left between theperipheries of the gold electrodes and ceramic disk to prevent shortcircuiting by arcing or otherwise around the edge of the ceramic disk.However, if the piezoelectric elements are glued to the diaphragm 58 theborder is eliminated. The gluing technique, hereinafter described, ispreferred because the border 56 is not required, thus for a given sizeceramic disk the electrode size is increased.

The above mentioned thick film process is well known to those skilled inthe art; therefore, only a brief description need be given. The processconsists of spreading a gold paste to a thickness of 0.0007 inches overa 200-325 mesh nylon screen and drying at room temperature for to 15minutes, and then firing, i.e., heating to temperatures below 750Cthrough a conveyor type furnace with a 45 minute cycle to a peaktemperature of 750C for 5-10 minutes to obtain good electrode adherence.Temperatures above 750C are to be avoided to prevent damage to a leadcompound ceramic disk through evaporation of the lead. A suitable goldpaste is DuPonts Gold Conductor Composition 8115.

If nickel electrodes are desired they may be formed on the ceramic disksby vapor deposition techniques well known to those skilled in the art.

After forming the electrodes, the ceramic disk is polarized by a verypowerful d.c. electric field, e.g., 40,000 to 5 X 10 V/m. A dot or othermarking is used to designate the side the positive electrode wasattached for polarizing the ceramic disk. The polarized piezoelectricelements are then labeled, weighed and placed in a capacitance measuringinstrument and the weight and capacitance of each piezoelectric elementrecorded. The piezoelectric elements are then placed in bins accordingto their weight and capacitance.

Piezoelectric elements 44 and 46 are selected from the bins withsubstantially matching weights and capacitances for attachment to aweighed diaphragm 40 to form detector element 58. These measurements arefed into an automatic machine programmed to solve the above mentionedmathematical formulas to determine the mass and capacitance forpiezoelectric elements 48 and 50, and diaphragm 42, for the seconddetector element 58' required to match it to the first detector elementfor a noise cancellation hydrophone. Piezoelectric elements 48 and 50and diaphragm 42 having the required parameters or combination thereofto substantially match the response of the first detector are thenselected from the bins. Piezoelectric elements 44 and 46, andpiezoelectric elements 48 and 50, respectively, are then attachedselectively, either by soldering with an indium alloy or gluing withglue, GAl l 1, sold by Gulton Industries, to the diaphragms 40 and 42with the positive marked electrodes facing outwardly as indicated by thearrows in FIGS. 4, 5, and 6 to complete the detector elements 58 and 58for the hydrophone subassembly 22. Gluing is preferred over soldering inthat; the glue may be spread evenly at a desired thickness over a plate,portions of the glue, identical to the shape (circular) of theelectrode, cut by a cutter and applied to the negative electrode toattach the piezoelectric element to the diaphragm. Thereafter thedetector elements are stacked in a pressure exerting type holder and theglue cured for about two hours at room temperature. By gluing the massof the detector elements can be controlled and the possibility ofshorting between the piezoelectric elements and the diaphragm isalleviated. The detector elements are then weighed and theircapacitances measured. The weight and capacitance measured is recordedfor each detector element. If these parameters match substantially ahydrophone is fabricated as follows:

Electrical leads 60 and 62 are attached as shown in FIG. 5 to theelectrodes that are to form the innermost plates of the hydrophone, andthe detector elements 58 and 58 mounted in the recesses (FIG. 4) formedin the ends 36 and 38 of the detector element support member or frame 34with the electrical leads 60 and 62 brought out through the detectorelement support member 34 (FIG. 3). Electrical leads 64 and 66 are; thenattached to the outermost electrodes as shown ini FIG. 5 to complete thehydrophone subassembly 22.

The hydrophone subassembly 22 (FIGS. 2, 3, and 4) is then mounted in thehousing 20 by positioning the retaining means 32 of the subassembly inline with the mounting means 30 of the housing in order that they willmate when the subassembly 22 is encapsulated with a suitable plasticsuch as, for example, polyurethane. The subassembly is then encapsulatedwith thicknesses of encapsulating material covering the major surfacesequal or selectively varied one to another to adjust the effective massof the piezoelectric elements. After encapsulation the electrical leads60 and 66 are attached to output terminals 26 and 28 (FIG. 2).

In another embodiment of the invention (FIG. 2A) the housing iseliminated. F IG. 2A depicts this embodiment with identical partsbearing the same numbers with a prime. Terminals 26 and 28 are terminals26 and 28 modified as shown and attached to leads 60 and 66'. Thehydrophone subassembly 22 is positioned in an encapsulating mold andselectively encapsulated; that is, the thickness of the encapsulatingmaterial on the major surfaces of the detector elements is controlled tochange the piezoelectric elements response to acceleration noise forcancellation. The encapsulation step completes the fabrication of thehydrophone.

With the detector elements 58 and 58' electrically connected as abovedescribed for either embodiment of the invention, a series-parallelrelationship is established through which acceleration noise (FIG. 6)may be detected and cancelled one from the other while the pressure wave(FIG. 5) may be detected by both with- 2 5 out cancellation. Theabove-described acceleration noise cancellation is referred to as firstorder cancellation.

The hydrophone is then given a shake test to determine its response toacceleration noise and an acoustic lating material from the faces of thehydrophone by sanding.

Although hydrophones constructed in accordance with the first ordernoise cancellation embodiments of the invention under ideal conditionswill reduce acceleration noise output to zero, it will be apparent thatin actual use it will be difficult to achieve zero noise cancellation.Thus further acceleration noise cancellation is desirable and isprovided by what is referred to as second order acceleration cancelling.

Second order acceleration noise cancellation is acquired by subjectingeach hydrophone prior to its incorporation into the streamer or streamersection, to tests to determine capacitance, acceleration sensitivity,acoustic sensitivity, acoustic polarity, terminal to terminal resistanceand terminal to mount resistance. The

tests include subjecting the hydrophone to a known acceleration force todetermine its electrical response, i.e., its amplitude and phaseresponse as to the acceleration force. Thereafter, using theseparameters, the hydrophones are matched one with another or others toprovide second order noise cancellation. Here again the final thicknessof the encapsulating material of the hydrophones to be matched may bevaried by sanding to match the hydrophones one to another. It will beappreciated that the second embodiment of the invention 1 (FIG. 2A)lends itself more readily to adjustment of the encapsulating material.FIG. 9 shows two hydrophones 118 and coupled in parallel with theirpolarities such that their outputs (FIGS. 9A and B) are in phase andthus no cancellation occurs. FIG. 10 shows the same two hydrophones 1118and 120 with the polarity of hydrophone 120 changed by turning it aboutin the same streamer. Although the parallel connection is the same, theresponse to acceleration noise as shown in FIGS. 10A and B are out ofphase and subtract or cancel. In this arrangement the detectors responseto seismic waves is not affected. The above described arrangement isonly one of many that can be applied to any number of detectors whoseacceleration noise response may vary over a range of values withimproved results. Nevertheless, the ideal situation exists when the sumof the negative and positive voltage producing detectors approach zeroand the wavelength of the acceleration noise is large compared with thelength of the section. When the noise wavelength is equal to or lessthan the streamer section length the spacing of the hydrophones becomesa factor in positioning the hydrophones. Further, it will be understoodby those skilled in the art that the hydrophones can be rotated 90 andtransverse acceleration noise cancelled in a manner similar to thecancelling of the longitudinal noise described above. Thus, variouscombinations of transverse and longitudinal orientations and series andparallel wiring techniques or combinations thereof can be employed, asis well known in the art, to provide any desired noise cancellationdepending on knowledge of the magnitude and direction of theacceleration noise.

TABLE II lists the acceleration sensitivity measured in milliamps pergram (ma/g) for 30 hydrophones used in test cables. Case I represents anarrangement of the hydrophones without regard to acceleration polaritiesor the worst case; Case II represents the acceleration polarity matchingof hydrophones in accordance with an embodiment of the invention. Othermethods of evaluating the hydrophones include, for example, the productof the capacitance times the acceleration sensitivity.

TABLE II HYDRO- CASE I CASE II PHONE Acceleration SensitivityAcceleration Sensitivity (Unmatched) (Matched) l 3.1 3.1 2 3.5 -3.5 310.5 l0.5 4 12.0 12.0 5 9.0 9.0 6 8.5 8.5 7 8.5 8,5 8 I 1.0 -1 1.0 9 8.5*8.5 10 12.0 12.0 I l 9.0 9.0 12 9.0 9.0 Iii 7.5 7.5

TABLE II Continued HYDRO- CASE 1 CASE ll PHONE Acceleration SensitivityAcceleration Sensitivity (Unmatched) (Matched) 14 12.5 12.5 15 10.0 10.016 12.5 l2.5 17 27.5 27.5 18 30.0 30.0 19 25.0 25.0 20 20.0 20.0 21 13.513.5 22 15.5 15.5 23 21.0 21.0 24 31.0 3l.0 25 12.5 12.5 26 14.0 14.0 27l 1.0 1 1.0 28 12.5 12.5 29 15.0 15.0 30 21.0 21.0

NOTE: indicates acceleration polarity is reversed.

In the streamers constructed utilizing the hydrophones of TABLE ll, thefirst hydrophone was positioned 13.66 feet from one end coupling of thestreamer, the next 14 hydrophones were equally spaced throughout thenext 54.67 feet of the streamer, nol ydrophones were positioned withinthe center 27.33 feet of the streamer where the 16th hydrophone waspositioned, the remaining l4 hydrophones were equally spaced within thenext 54.67 feet of the streamer; and the other end coupling was spaced13.66 feet from the last hydrophone. The results of the trial runs areshown in the following chart in which the relative response of thestreamers to acceleration forces are plotted on the ordinate axis andthe wave number X in cycles per foot and the corresponding wavelength Ain feet per cycle. The chart clearly shows the improved performancewhere the WORST CASE POLARITY MATCHED Relative Response in Decibels A,

Frequency in Cycles Per Second -60 l l l l 1 1 I l 1 Wave Number K inCycles Per Foot llllnlinlilnulilimlilllil l l 10 5 3 2 1.5 1.0.9 ,B .7.6 .5

Wave Length A in 100 Feet Per Cycle ing substantially equal response topressure waves the improvement comprising:

a. determining the geometric and physical property parameters andelectrical parameters of a first detector element and its response to aselected acceleration noise wave and a pressure wave;

b. selecting a second detector element having geometric and physicalproperty parameters and electrical parameters which when combinedelectrically with the first detector element to form a hydrophone havingsubstantially matching responses to acceleration noise waves andpressure waves;

c. mounting said first and second detector elements in a frame such thatthey are coupled in an acceleration noise cancelling mode; and

d. selectively varying geometric and physical property parameters of atleast one of the detector elements after the mounting step and duringfurther construction of the hydrophone to vary the response of thehydrophone to acceleration noise and pressure waves to produce anhydrophone having substantially acceleration noise cancelling responseand duplicate pressure wave response.

2. A method for producing a pressure sensitive hydrophone comprising:

a. determining the geometric and physical property parameters andelectrical parameters for a plurality of piezoelectric elements;

b. selecting a first pair of piezoelectric elements having substantiallymatching geometric and physical property parameters and electricalparameters;

0. selectively mounting the pair of piezoelectric elements on opposingsides of a first support member of known weight and compliance to form afirst detector element;

d. determining the geometric and physical property parameters andelectrical parameters of the first detector element;

e. selecting a second pair of piezoelectric elements having geometricand physical property parameters and electrical parameters substantiallymatching those of said first pair;

f. selectively mounting the second pair of piezoelectric elements onopposing sides of a second support member having propertiessubstantially matching those of the first support member, to form asecond detector element having geometric and physical propertyparameters and electrical parameters substantially matching those of thefirst detector element;

g. determining the geometric and physical property parameters andelectrical parameters of the second detector element;

h. selectively mounting the first and second detector elements inopposing ends of a hydrophone frame;

i. selectively electrically coupling the detector elements in anacceleration noise cancelling mode; and

j. selectively varying geometric and physical property parameters of atleast one of the detector elements by selectively encapsulating theframe and detector elements thereby producing a hydrophone havingsubstantially acceleration noise cancelling response and duplicatepressure wave response.

3. A method according to claim 2, wherein the step of selectivelyencapsulating the frame and detector elements includes selective removalof encapsulating material covering the detector elements of thehydrophone.

4. A method for producing a pressure sensitive hydrophone according toclaim 2 wherein the second detector element is off set to the firstdetector element through geometric and physical property parameters andelectrical parameters including for the electrical side the voltageoutput (E of each detector element, the capacitance (C of eachpiezoelectric element; and for the mechanical side the pressure force(F) owing to acoustic forces and a velocity force (V) owing toacceleration and deceleration forces, compliance (C of each detectorelement mounting, mass (M of each piezoelectric element and diaphragm (Mturns ratio (N representing electromechanical transformer action of eachpiezoelectric element and the mechanical impedance (Z) of each detectorelement where x equals 1, 2, 3 or 4 indicating the particularpiezoelectric elements and p equals 1 or 2 indicating the particulardiaphragm.

5. A method for producing a pressure sensitive hydrophone according toclaim 4 wherein the first and second detector elements are off set byfeeding the known geometric and physical property parameters andelectrical parameters of a first piezoelectric element of the firstdetector element into an automatic machine programmed to determinematching parameters for the remaining piezoelectric element and thesecond detector element in accordance with the following relationships:

2 omar 111 1 l CIIIICIIIL'. ,3 m! um! where,

"111 m1 m2)/( 1+ 2 n) for the second detector element "112 m3 m4)/( 3 4n) and for the combined open-circuit output of the hydrophone oc iz 01 no2)/( t2 n) where

1. In a method for producing a pressure sensitive hydrophone having apair of detector elements coupled in an acceleration noise cancellingmode while maintaining substantially equal response to pressure wavesthe improvement comprising: a. determining the geometric and physicalproperty parameters and electrical parameters of a first detectorelement and its response to a selected acceleration noise wave and apressure wave; b. selecting a second detector element having geometricand physical property parameters and electrical parameters which whencombined electrically with the first detector element to form ahydrophone having substantially matching responses to acceleration noisewaves and pressure waves; c. mounting said first and second detectorelements in a frame such that they are coupled in an acceleration noisecancelling mode; and d. selectively varying geometric and physicalproperty parameters of at least one of the detector elements after themounting step and during further construction of the hydrophone to varythe response of the hydrophone to acceleration noise and pressure wavesto produce an hydrophone having substantially acceleration noisecancelling response and duplicate pressure wave response.
 2. A methodfor producing a pressure sensitive hydrophone comprising: a. determiningthe geometric and physical property parameters and electrical parametErsfor a plurality of piezoelectric elements; b. selecting a first pair ofpiezoelectric elements having substantially matching geometric andphysical property parameters and electrical parameters; c. selectivelymounting the pair of piezoelectric elements on opposing sides of a firstsupport member of known weight and compliance to form a first detectorelement; d. determining the geometric and physical property parametersand electrical parameters of the first detector element; e. selecting asecond pair of piezoelectric elements having geometric and physicalproperty parameters and electrical parameters substantially matchingthose of said first pair; f. selectively mounting the second pair ofpiezoelectric elements on opposing sides of a second support memberhaving properties substantially matching those of the first supportmember, to form a second detector element having geometric and physicalproperty parameters and electrical parameters substantially matchingthose of the first detector element; g. determining the geometric andphysical property parameters and electrical parameters of the seconddetector element; h. selectively mounting the first and second detectorelements in opposing ends of a hydrophone frame; i. selectivelyelectrically coupling the detector elements in an acceleration noisecancelling mode; and j. selectively varying geometric and physicalproperty parameters of at least one of the detector elements byselectively encapsulating the frame and detector elements therebyproducing a hydrophone having substantially acceleration noisecancelling response and duplicate pressure wave response.
 3. A methodaccording to claim 2, wherein the step of selectively encapsulating theframe and detector elements includes selective removal of encapsulatingmaterial covering the detector elements of the hydrophone.
 4. A methodfor producing a pressure sensitive hydrophone according to claim 2wherein the second detector element is off set to the first detectorelement through geometric and physical property parameters andelectrical parameters including for the electrical side the voltageoutput (Eo) of each detector element, the capacitance (Cex) of eachpiezoelectric element; and for the mechanical side the pressure force(F) owing to acoustic forces and a velocity force (V) owing toacceleration and deceleration forces, compliance (Cmx) of each detectorelement mounting, mass (Mx) of each piezoelectric element and diaphragm(Mrp), turns ratio (Nx) representing electromechanical transformeraction of each piezoelectric element and the mechanical impedance (Z) ofeach detector element where x equals 1, 2, 3 or 4 indicating theparticular piezoelectric elements and p equals 1 or 2 indicating theparticular diaphragm.
 5. A method for producing a pressure sensitivehydrophone according to claim 4 wherein the first and second detectorelements are off set by feeding the known geometric and physicalproperty parameters and electrical parameters of a first piezoelectricelement of the first detector element into an automatic machineprogrammed to determine matching parameters for the remainingpiezoelectric element and the second detector element in accordance withthe following relationships: